Determination Of The Real Part Of The Eta '-nb Optical Potential

The excitation function and momentum distribution of eta' mesons have been measured in photoproduction off Nb-93 in the energy range of 1.2-2.9 GeV. The experiment has been performed with the combined Crystal Barrel and MiniTAPS detector system, using tagged photon beams from the ELSA electron accelerator. Information on the sign and magnitude of the real part of the eta'-Nb potential has been extracted from a comparison of the data with model calculations. An attractive potential of -(41 +/- 10(stat) +/- 15(syst)) MeV depth at normal nuclear matter density is deduced within model uncertainties. This value is consistent with the potential depth of -(37 +/- 10(stat) +/- 10(syst)) MeV obtained in an earlier measurement for a light nucleus (carbon). This relatively shallow eta'-nucleus potential will make the search for eta'-nucleus bound states more difficult.